I had a poke around in the code using the browser developer tools and found something, but am unsure if it's the correct way to do it.
I assumed that "main toolbar" meant Layout > Topic Page Buttons. I noticed the existing ones had an OnClick event like "EVT_SHOW_FILTERS". So I searched using the browser developer tools and found a list of capitalised text that included that entry in the topicpage.js file. I found another one called "TOGGLE_FAVOURITE_TOPIC" and used that for the OnClick event of a new button. It seems to set and unset the favourite topic correctly. So you could use the devtools to look at the list and determine the other buttons needed.
But for a proper answer I think you would need to contact Adobe directly. See the following page for support contact options. The email address is recommended as it goes to a dedicated Robohelp team.
https://helpx.adobe.com/contact/enterprise-support.other.html#robohelp
Oh and then I did a search for another one of the OnClick options already present and found this which mentions remove highlight, and print.
https://community.adobe.com/t5/robohelp-discussions/add-custom-quot-topic-page-button-quot-to-exit-close-help/td-p/13388860